1. Field of the Invention
The present invention relates to a transmission apparatus, particularly an automotive transmission, and more particularly a continuously variable automotive transmission.
2. Description of the Prior Art
There are several types of known automotive transmissions which do not utilize gears. Amongst these are belt type, cam type, ring-corn type, toroidal type, and a differential with a combination of a variable oil pump and an oil motor.
Belt type transmissions alter the output/input rotation ratio by changing the radius ratio between a set of two V-pulleys. Such a mechanism necessitates frictional force between the belt and the pulleys. This frictional force exists independent of rotation of the pulleys, and exacts a price both in resistance (and hence efficiency) and a reduced lifetime of component parts. Additionally, this sort of transmission cannot provide a zero or nearly zero rotation ratio by virtue of its design.
A cam type transmission, which combines a cam with a one-way clutch, has many disadvantages. These disadvantages include a relatively narrow rotation ratio from zero to approximately 1/4; relatively large resistance due to the existence of many friction points in links and one-way clutches; a great many structural elements; and a rather large size.
With respect to the ring-corn type and the toroidal type, the disadvantages include the requirement that structural elements be manufactured with high precision, as power is conveyed by friction caused by subtle elasticity or an automatic pressure device between an input disk, an output disk, and a tapered roller or idler roller. Also, conveyable torque is relatively low since a very limited contact area must be used in order to avoid brake effect, which will occur with a wider contact area. Finally, an output rotation of zero is unobtainable because of structural limitations and/or abrasion problems.
A typical automotive transmission, using a differential gear system, performs a transmission function, including a zero output ratio, by using three shafts of the differential gear to an input shaft, an output shaft, and a control shaft. The control shaft is connected to an oil pump, which includes an adjustable control valve.
This type of differential gear transmission suffers from a great many structural elements, a complicated and large structure due to a combination of the differential gear with the oil pump, and difficulty with torque conveyance when the output ratio is lowered, because the control valve is loosened to lower the output rotation. When the control valve is thus loosened, the oil in the oil pump flows freely, and thus conveys no power.
An automotive transmission which combines an oil pump with an oil motor also has the drawback of a large number of elements resulting in a heavy and bulky structure, given that it is a combination of two almost identical units, as each can be used as a pump.